The present invention relates to transparent multiplexer/demultiplexers (TMUX) for use with a high speed connection between two points. More specifically, the invention relates to a method for encoding error counts for each tributary signal being transmitted through the TMUX.
High speed links between two points have been in use in the communications industry for the past few years. The SONET standards ANSI T1.105 and Bellcore GR-253-CORE define the physical interface, optical line rates known as optical carrier signals, a frame format, and an operation, administration, maintenance, and provision ways protocol. These SONET standards can be used with a transparent multiplexer/demultiplexer (TMUX) at both ends of a high speed point-to-point link. By multiplexing a number of signals (each called a tributary or trib) at one end onto a single link and demultiplexing the resulting signal at the other end, an effective multiplexing of the link capacity can be achieved.
Ideally, such a TMUX needs to effectively provide full transparency for each trib. Such full transparency means that any errors in a trib at one end must survive the multiplexing/demultiplexing process. One approach detects B1 errors for each trib and if the error is greater than a specific number, generate an estimate of that error rate. This estimate is then encoded and transmitted through the TMUX to the other end. At this other end, the encoded estimate is extracted and decoded. The estimated error rate is then duplicated for this bit by readjusting the parity bits of the signal prior to transmission from the trib. Such estimation, under some circumstances depending on the base encoding method, may not be as accurate as desired.
Furthermore, if the transport of the encoded error rate is accomplished by using a single unused byte in the overhead generated for the TMUX, the loss of this byte may lead to faults.
From the above, there is therefore a need for an encoding scheme which may be used with a TMUX system. Such an encoding scheme must be accurate and, at least to some measure, fault tolerant of lost or dropped bytes.
The present invention overcomes the shortcomings of the prior art by providing a method of encoding bit error rates using a two-tiered system. If the bit error rate is less than a threshold value, the bit error rate is directly encoded in binary into substantially equal portions. However, if the error rate is equal to or greater than the threshold value, the error rate is encoded into equal valued amounts using a common step value. A remainder amount is also formulated to account for any remainders. Multiplying the equal valued amounts by a common base value and then adding it all up along with the remainder amount results in a value which is at worst, within 0.2% of the original bit error rate value.
For fault tolerance, instead of only using a single byte to encode the error rate, additional unused bytes may be used. This provides a level of fault tolerance in that if there are 10 unused bytes and a single byte is lost, 90% of the encoded data (the encoded error rate) is still accessible.
In a first aspect the present invention provides a method of preserving error counts for a plurality of tributary signals into a transparent multiplexer/demultiplexer (TMUX) said tributary signals being transparently transported over a high data rate span, the method comprising:
at a first end of said span:
a1) detecting a raw error count for each incoming tributary signal;
b1) encoding said raw error count into an encoded error count;
c1) inserting said encoded error count in specific unused bytes of successive TMUX messages for transport to a second end of said span;
at the second end:
a2) extracting said encoded error count from specific unused bytes of successively received TMUX messages transported from the first end;
b2) decoding said encoded error count into the raw error cont;
c2) adjusting portions of the tributary signal received from the first end such that said tributary signal has an error count substantially equal to said raw error count.
In a second aspect the present invention provides a method of encoding a raw error count of a tributary signal for use with a transparent multiplexer/demultiplexer said method comprising:
determining said error count
if said raw error count is less than a threshold value, encoding in binary said raw error count into one remainder portion and a number of equally valued portions such that a sum total of all the portions is equal to said raw error count;
if said raw error count is greater than said threshold value, encoding in binary said raw error count into one remainder portion and a number of equally valued non-remainder portions such that a sum of
all non-remainder portions multiplied
by a base value and
the remainder portion is substantially equal to the raw error count.
In a third aspect the present invention provides a method of encoding a raw error count of a tributary signal for use with a transparent multiplexer/demultiplexer, said method comprising:
determining said raw error count;
if said error count is less than a threshold value, encoding in binary said error count into a remainder portion and a number of equally valued portions such that a sum total of all the portions is equal to said raw error count;
if said error count is greater than the threshold value, encoding in binary said error count into one remainder portion and a number of equally valued non-remainder portions such that a sum of
all non-remainder portions multiplied by a base value; and
the remainder portion multiplied by the base value is substantially equal to the raw error count.
In a fourth aspect the present invention provides a method of encoding a raw error count of a tributary signal for use with a transparent multiplexed/demultiplexer, said method comprising:
determining said raw error count;
dividing said error count into a number of equally valued portions and a remainder portion
such that a specific condition is true, said condition being chosen from the group comprising:
a sum total of all the portions is equal to said error count;
a sum total of all the portions multiplied by a base value is substantially equal to said error count; and
a sum total of the equally valued portions multiplied by the base value and added to the remainder portion is substantially equal to the error count.